Image keeping



Oct. 1962 L. E. WALKUP ETAL 3,057,275

IMAGE KEEPING 2 Sheets-Sheet 1 Filed Oct. 29, 1958 FIG. 2

INVENTORS' Lewis E.Walkup John F. Byrne QQQ ATTORNEY Oct. 9, 1962 L. E.WALKUP ETAL 3,05

IMAGE KEEPING 2 Sheets-Sheet 2 Filed Oct. 29, 1958 2 6 V IIIIIIFGIIIINVENTORS' Lewis E.Walkup John F. Byrne m @QM A T TORNE Y United StatesPatent Ofifice Patented Oct. 9, 1962 3,057,275 IMAGE KEEHNG Lewis E.Walkup and John F. Eyrne, Coiumhus, Ohio, assignors, by mesneassignments, to Xerox Corporation, a corporation of New York Filed (Bot.29, 1958, Ser. No. 779,543 6 Qlairns. (Cl. 951.7)

This invention relates to the preservation of electrostatic latentimages in xerography and related arts.

In xerography and related arts it is customary to form an electrostaticlatent image by one of several methods. According to one method, anelectric charge is first placed on the surface of a photoconductiveinsulating member which charge is then selectively dissipated byexposing the photoconductive insulating member to a pattern of light andshadow in image configuration. In a second method, a preexistingelectrostatic charge pattern on a photoconductive insulating member istransferred to an ordinary insulator not having photoconductiveproperties. In still another method a charge pattern is formed on aninsulating member through the action of high voltage pulses applied toadjacent shaped conductive electrodes. Many other methods for formingelectrostatic latent images are known and may be used in connection withthe present invention. One such additional method will be discussedlater. These electrostatic latent images, however formed aresubsequently made visible through known xerographic developmenttechniques or otherwis used, as by reading the charge pattern with anelectrometer or the like.

It is always essential that the member which carries the electrostaticimage have sufiicient resistivity to retain the charge pattern until itis to be utilized. Charge patterns usually decay in an exponentialfashion with the characteristic decay time being given by the equation1= e where 1- is the time constant, p is the resistivity of the imagebearing member and e is the dielectric constant of the member, allquantities being expressed in MKS units. The available resistivities insuitable image bearing members have heretofore limited the length oftime during which it has been possible to retain an electrostatic image.This has been particularly true with photoconductive insulatingmaterials where it has generally been necessary to develop the chargepattern very soon after its formation. Ordinary insulating materials canbe obtained with resistivies higher than those of most photoconductiveinsulators, but here also the resistivity is often inadequate to supportthe charge for a desired length of time.

It is accordingly an object of this invention to pro vide means andmethods for preserving electrostatic latent images for extended periodsof time.

It is a further object of this invention to provide xerographic cameraapparatus which permits image development to be carried out at a timeand place remote from exposure.

It is a still further object of this invention to provide self-containedxerographic camera apparatus having no requirement for integral chargingor development means.

The invention will be further described in connection with the figuresin which:

FIG. 1 shows one form of xerographic apparatus embodying the invention;

FIG 2 shows another form of xerographic apparatus embodying theinvention;

FIG. 3 shows still another form of xerographic apparatus embodying theinventtion;

FIG. 4 is a detailed view of an element of the apparatus of FIG. 3, and

FIG. 5 is an alternative embodiment of the apparatus of FIG. 3.

FIG. 1 shows a xerographic camera capable of recording three separatexerographic color separation images with a single exposure. It is morefully described in copending patent application Serial No. 581,912, nowPatent No. 2,962,374. As illustrated, there is a camera housing,including, for example, bellows 10 supporting exposure means such as alens 11 with suitable shutter mechanism (not shown) as is conventionalin the camera art. Positioned to make a light shield with the bellows isa rear panel 12 operably mounted on a hinge 13 and secured by a catch14. Mounted on a hinge 15 adjacent to the back panel is a xerographicplate 16 adapted to be positioned closely adjacent to the back panel.Xerographic plate 16 has a first photoconductive layer sensitive to afirst color facing the lens 11, and a second photoconductive layersensitive to a second color on the opposite side. Positioned in front ofplate 16 is a transparent conductive electrode 17. On the inside surfaceof rear panel 12 is a photoconductive layer 18 sensitive to a thirdcolor. Passing between this photoconductive layer and the xerographicplate 16 is a first insulating web 19 fed from a conductive feed roll 20and a second web 21 fed from feed roll 22. The surface of web 19 whichcontacts web 21 is coated with a transparent conductive coating. A thirdweb 23 held on a conductive feed roll 24 passes between the xerographicplate 16 and electrode 17. The surface of web 23 which contactselectrode 17 is coated with a transparent conductive coating. Webs 19,21 and 23 pass out of the camera housing through a slot at the bottomand are wound up on take-up spools 25, 26 and 27 respectively. Thesetake-up spools can be operated in unison byoperating handle 28 which isattached to shaft 29 which is journaled in supports (not shown) andcarries along its length three worms 30, 31 and 32, which engage wormwheels 33, 34 and 35 respectively which in turn are mounted on take-upspools 25, 26 and 27 respectively. In addition, there is a roll 36 ofconductive foil which is mounted so that the foil is interleaved withWeb 21 as it is rolled up on take-up spool 26. As is shown in thefigure, suitable power supply means are operably connected to the rearpanel 12, conductive electrode 17 and to the conductive surfaces of thewebs through conductive feed rollers 24) and 24.

In use and operation the camera of FIG. 1 is suitably aimed and focusedand the shutter mechanism is released while operating electricpotentials are applied to the electrode members. This results in theformation of developable electrostatic xerographic latent images on thethree webs. The image bearing webs are then rolled up on take-up spools25, 26 and 27 by operating handle 28. This puts the camera in conditionfor taking another exposure. Many such exposures can be taken and whenthe webs are completely rolled up on the take-up spools the spools maybe removed from the camera and carried to suitable developing apparatuswhere the charge patterns thereon may be made visible.

As the insulating webs emerge from the camera they carry on theirsurfaces electrostatic charge patterns which may have potentials rangingup to several hundred volts. However, as webs 19 and 23 are rolled ontotheir respective takeup stools the image bearing surface of each webcomes into contact with the conductive coating on the opposite side ofthe web. The potential on the surface of the web before it is rolled upis given by the relation where V is the voltage, Q is the charge perunit area and C is the capacitance per unit area. This capacitance isessentially that of the web itself which separates the charge on one ofits surfaces from the conductive coating on the other surface. As thecharge carrying surface is rolled against the conductive coating,however, the capacitance as seen at the charge carrying surface ismultiplied many times because the conductive coating on the next turn onthe web is in virtual contact with the charge carrying surface layerrather than being separated therefrom by the thickness of the web.Accordingly, the potential at the charge carrying surface of the web isreduced nearly to Zero. As will be discussed subsequently, the charge onthe web does not jump the nominal gap separating the charge from theadjacent conductive coating. As a matter of fact, this nominal orinfinitesimal gap is found to have very excellent insulating qualities.The situation involved in rolling up webs 19' and 23 may be likened tothat which occurs when a large capacitor with a very long time constantis connected in parallel with a smaller charged capacitor with a shortertime constant. In that situation most of the charge on the smallcapacitor would be transferred to the large capacitor and the timeconstant of the combination would be largely determined by the timeconstant or resistance of the larger capacitor. Obviously, the largercapacitor corresponds to the capacitance per unit area found betweenadjacent turns or layers as webs 19 and 23 are wound up and the smallercapacitor corresponds to the capacitance per unit area through webs 19and 23. When each web is subsequently unrolled the capacitance at thecharge carrying surface is restored to its former low value and thecharge pattern reappears essentially as it was before the web was rolledup. Since web 21 does not have a conductive coating, a thin conductivelayer such as a metal foil 36 is interleaved in web 21 as the web isrolled up. This accomplishes the same result as the conductive coatingson webs 19 and 23.

FIG. 2 shows a different type of xerographic camera apparatus embodyinga different form of the present invention. Again there is shown a camerabody having a bellows 10, a lens 11 and a back 12 supported on hinge 13and held closed by catch member 14. Mounted inside the camera is a feedroller 46 carrying a supply of a xerographic web plate 41. Plate 41 maytake several forms. It may comprise a thin metal foil coated with alayer of vitreous selenium or other photoconductive material, or it maycomprise a web of foil coated with a lacquer coating of aphotoconductive pigment such as Zinc oxide dispersed in a resin binder.In some cases the metal foil may be replaced by a strip of slightlymoistened paper. The photoconductive insulating layer should face thelens. Various other combinations of the photoconductive insulatingmaterials and support materials are known in the art and may also beembodied. A corona charging device 42 is mounted within the camera toapply an electrostatic charge to plate 41 as it passes chargfrom thecamera.

To operate the camera switch 44 is closed and a suitable length of plate41 and foil 46 are withdrawn from the camera. The length withdrawnshould be substantially comparable to the length of the focal plane ofthe camera. Plate 41 is electrostatically charged as it passes beneaththe corona charging device 42 and there results a charged length ofplate 41 lying in the focal plane of the camera. Switch 44 may then beopened and, assuming that the first exposure is yet to be made, plate 41and foil 46 are severed at knife 47 and discarded. The camera is thenaimed and focused and the shutter mechanism is released to project apattern of light and shadow onto plate 41 and thereby to form anelectroyshutter associated with lens 11.

. for use.

static latent image thereon. After the exposure is completed the exposedlength of plate 41 is withdrawn from the camera. Normally, switch 44will be closed while plate 41 is being withdrawn in order to prepare thecamera for a second exposure. As plate 41 is withdrawn, an adherentcovering of foil 46 is withdrawn with it. Foil 46 will adhere tightly tothe surface of plate 41 by electrostatic attraction. Thus, once theleading edge of foil 46 is brought into contact with plate 41, foil 46will continue to feed from the supply roll 45 into contact with plate 41whenever a length of plate is withdrawn from the camera. After theexposed length of plate is withdrawn, the plate together with its foilcovering may be severed by tearing against knife 47 to form a singlepiece of electrostatic latent image bearing xerographic plate which mayconveniently be stored. When a number of such pieces of plate have beenexposed, they may be carried to a suitable development apparatus andthere developed after removal of the adherent foil covering.

As plate 41 comes into contact with foil 46 upon removal from thecamera, the capacitance at the charge bearing surface of the plate isvery greatly increased and the potential at the plate surface is greatlyreduced. Since the capacitance appearing between the plate 41 and web 46is very large and since this capacitance is associated with a very highresistance or a very long time constant, in spite of apparent contactbetween the conductive foil and the plate, the charge pattern on plate41 can be preserved for very long periods of time. Thus, there is noneed to develop each exposure as soon as it is made, but instead manyexposures can be made and then developed later at the operatorsconvenience. For this reason it is not necessary to carry conventionalheavy and bulky development apparatus with the camera. When the adherentconductor foil is removed prior to development, the electrostatic chargepattern on plate 41 is restored substantially to the form and magnitudeit had before the foil was applied.

Foil 46 also performs an entirely different, but equally usefulfunction. Since plate 41 is sensitive to light, it would ordinarily benecessary to keep it in darkness at all times between exposure anddevelopment to prevent the electrostatic latent image from beingdestroyed by light. However, where an opaque foil 46- is used asdescribed herein, the photosensitive surface of the plate is protectedfrom light, and from abrasion as well, at all times after it iswithdrawn from the camera. Thus, the exposed pieces of plate can behandled with the same ease as ordinary pieces of paper and no speciallight tight plate holders or carrying containers are required.

FIG. 3 shows a schematic representation of a simplified xerographiccamera embodying the present invention. The camera of this figure, likethose of FIGURES 1 and 2, includes a bellows 10, lens 11, rear panel 12,hinge 13 and catch 14. It need not, however, have a conventional Mountedwithin the camera is a feed roller 50 carrying a xerographic web plate51 which is wound up on a take-up spool 52 also within the camera. Webplate 51 may be any of the types already discussed in connection withweb plate 41 of FIG. 2. As in FIG. 2, the photoconductive insulatinglayer should face the lens. Web plate 51 supplied from feed roller 50 isin a precharged condition. Thus, at some prior time the length of webplate 51 may have been carried past a charging device, such as a coronacharging device and then wound up on feed roll 50 Where, in accordancewith the present invention, it will retain its charge for an extendedperiod of time. Feed roller 50 is then mounted inside the camera and webplate 51 threaded onto take-up spool 52 to prepare the camera Rollers 53and 54 are also mounted inside the camera adjacent to web plate 51 andcarry between them a conductive web 55. This web which may preferablycomprise a thin strip of metal such as brass shim stock, contains atleast one aperture which is uniform in its dimension parallel to thelength of the web and extends nearly, but not quite, across the width ofthe web and which can be wound up on either roller 53 or 54. Thus, web55 together with rollers 53 and 54 constitute a focal plane shutter asis well known in the photographic art, and this shutter may be providedwith a conventional operating mechanism, not shown in this figure. Thereis also provided a glass plate 56 with releasable pressure means, notshown in this figure, to urge conductive web 55 against Web plate 51.Light shields 57 are also provided to prevent light from lens 11reaching web plate 51 except through the aperture in conductive Web 55.Between picture-taking operations, the aperture in conductive web 55 isrolled onto either roller 53 or 54 and web plate 51 is protected, inaccordance with this invention, against loss of charge where it isrolled up on feed roller 50, wher it is contacted by conductive web 55,and where it is rolled up on take-up spool 52. The small unprotectedregions of web 51 between rollers 50 and 53 and 54 and 52 may lose theircharge, but this is unimportant since these regions are merely used toseparate successive exposures.

FIG. 4 is a perspective view of certain portions of the camera of FIG. 3together with certain additional features not shown in FIG. 3. Rollers50, 52, 53 and 54; xerographic plate 51; and glass plate 56 areidentical with the correspondingly numbered lements in FIG. 3. Certainjournaling means for the rollers have been omitted for simplicity. Glassplate 56, as shown, is mounted on a frame 60 by means of leaf springs61. Frame 60 in turn is mounted on shaft 62 which is journaled in blocks63. A coil spring 64 is provided on shaft 62 to bias the shaft in adirection to apply downward pressure on glass plate 56. Blocks 63 aremounted on shutter operating mechanism 65. Mechanism 65, which is shownin schematic form, is the usual type of mechanism found on cameras withfocal plane shutters. It includes a button 66 for tripping the focalplane shutter a well as a knob 67 which when rotated simultaneouslywinds a predetermined length of Xerographic web plate 51 onto take-upspool 52 as well as acting on rollers 53 and 54 to rewind the shutter.As is common in focal plane cameras, knob 67 is ineffective until it hasbeen depressed. Also attached to the shutter operating mechanism 65 area pair of blocks 68 in which is journaled a shaft 69 which is connectedby a system of links 70 to shaft 62. One outboard end of shaft 69 hasmounted therein a radial pin 71 while the other end has a pin 72. Arocking lever 73 is mounted between button 66 and pin 71 so that whenbutton 66 is depressed the rocking lever 73 pushes against pin 71, thusrotating shaft 69, and through link 70 shaft 62 as well, thereby liftingglass plate 56. A similar rocking lever 74 is mounted between windingknob 67 and pin 72 so that depressing knob 67 also raises glass plate56.

To make an exposure with the camera of FIGURES 3 and 4, button 66 isdepressed, thereby releasing the pressure on glass plate 56 allowingconductive web 55 to separate slightly from web plate 51 andsimultaneously operating the focal plane shutter mechanism to permit theexposure aperture in conductive web 55 to pass from one of rollers 53 or54 to the other, thereby exposing Web plate 51 and forming anelectrostatic latent image thereon. Next, winding knob 67 is depressedand rotated. This Winds a length of plate 51 from feed roller 50 ontotakeup spool 52, rewinds the focal plane shutter and removes th pressurefrom glass plate 56 while these operations take place. When knob 67 isreleased, pressure is reapplied to glass plate 56 and the camera isagain in a quiescent condition, ready either for a long delay or forimmediate additional picture-taking.

When the entire length of Web plate 51 has been exposed, take-up spool52 with web plate 51 wound thereon may be removed from the camera andthe electrostatic latent images on the plate may be developed later andelsewhere at the convenience of the operator. There is thus provided aportable xerographic camera which requires no electrical power supplyand no charging or developing apparatus or other heavy, bulky,complicated equipment. This camera may be used in the same manner as anordinary photographic camera; i.e., the operator may purchase analready-charged supply of web plate 51 and after it has been exposed hemay simply sent it away to be developed. It has the additional benefitof producing positives immediately without an intermediate negative andthe positive, if desired, may be used as a master to producexerographically any additional number of positive prints.

A simplified form of the camera shown in FIGURES 3 and 4 may beconstructed as illustrated in FIG. 5 by omitting elements 53, 54 and 55,comprising the focal plane shutter, and substituting instead aconventional between-the-len shutter 80. If such a substitution is madeglass plate 56 should be replaced by a glass or other transparent plate81 having a transparent conductive coating 82, such as tin oxide, on thesurface thereof adjacent to web plate 51, and a connecting linkage 83should be provided between button 66 and the between-the-lens shutter.In such a camera, as in that of FIGURES 3 and 4, glass plate 56 or itsequivalent plate 81 exerts a pressure against web plate 51 at all timesexcept during exposure or film advance. Thus, the conductively coatedelement substituted for glass plate 56 performs the ame chargepreservation functions as does the conductive Web 55 in FIGURES 3 and 4.

While the invention has been described up to this point in terms ofmechanical apparatus, it is apparent that it can also be carried outequally well by manual means. Thus, by way of example, rather thanlimitation, where a conventional rigid xerographic plate i charged andexposed a conductive member may be manually placed in contact with theimage bearing surface of the plate in order to preserve theelectrostatic latent image for future use.

While the invention has previously been described solely in terms ofplacing a conductive member directly into contact with an electrostaticimage bearing member, it is also possible and within the scope of theinvention to very slightly separate the conductive member from theelectrostatic image bearing member. This may conveniently be done, forexample, by applying a very thin layer of insulating lacquer to theconductive member before it is placed into contact with the surfacecarrying the charge pattern. In order to achieve the desired result ofimage preservation, however, it is necessary both that the insulatinglayer which separates the conductive member from the charge carryingsurface should be substantially thinner than the insulating materialwhich carries the charge pattern and also that the insulating lacquer orother insulating material have a very high resistivity. In general, theuse of such lacquers or other insulating spacers is neither necessarynor desirable in carrying out the invention.

It is apparent that the working of this invention depends upon the factthat when a conductive member is placed in contact with a chargecarrying insulator no charge will flow between the insulator and theconductor. The conditions under which charge will not flow are morefully set forth in copending patent application Serial No. 718,247. Inaccordance with the teachings set forth therein, no transfer of chargewill take place in connection With this invention where theelectrostatic latent images have potentials on the order of severalhundred volts as is the usual situation, and where the appliedconductive member such as foil 36 or 46 is at the same potential as theconductive coating or backing of the insulating or photoconductiveinsulating material which carries the electrostatic charge pattern. Inaccordance with the teachings of the above referenced application, itmay be necessary to apply a potential to the conductive member where itis desired to preserve very high potential charge patterns havingpotentials of the order of about 700 volts or greater. It is obviouslynot feasible then in that situation to use the apparatus of FIG. 1. Inthe apparatus of FIG. 2, however, it is quite simple to apply thedesired voltage to foil 46 through supply spool 45. Where the use of avoltage on the conductive member is indicated this voltage should beapplied when the conductive member is brought into contact with thecharge bearing surface and when it is separated therefrom, but need notbe maintained at other times.

What is claimed is:

1. A xerographic camera including means to mount a multi-exposure rollof precharged xerographic plate, means to mount a take-up spool, meansto incrementally advance the plate through a focal plane to the take-upspool, a focal plane shutter positioned adjacent to the focal plane andincluding a conductive blind windable between two parallel spaced-apartrollers, and means to urge the portion of said blind lying between saidrollers against the xerographic plate except during exposure and plateadvance.

2. A xerographic camera comprising:

(a) a light-tight enclosure,

(b) an optical system,

(c) means to mount a roll of web material having an insulating layercoated on a conductive support material within said enclosure,

(d) means defining a path of advancement of said web material through afocal plane of said optical system so that the insulating layer facesthe optical system,

(e) means to position an electrically conductive member in contact withsaid insulating layer after formation of a latent electrostatic imagethereon,

(7) means to provide a conductive connection between said electricallyconductive member and said conductive support material in order toreduce the elec trical forces tending to dissipate said image.

3. A xerographic camera comprising:

(a) a camera housing,

(b) an exposure means,

() a xerographic plate having a photoconductive layer,

(d) a conductive surface positioned adjacent to said xerographic plate,

(2) means for feeding an insulating web between said xerographic plateand said conductive surface,

(f) means to apply an electrical field across said photoconductive layerand said insulating web during exposure to form an electrostatic latentimage on said web,

(g) take-up means for rolling up said web, and

(11) means to interleave a thin conductive foil in said web as it isrolled up in order to reduce dissipation of the electrostatic latentimage.

4. A xerographic camera apparatus comprising:

(a) a camera housing,

([7) a lens,

(0) feed means for feeding a xerographic web plate having aphotoconductive insulating layer through the focal plane of said lens,

(d) electrostatic charging means for applying an electrostatic charge tosaid layer as it enters said focal plane, and

(e) means to feed a web of thin conductive foil into continuous contactwith said layer as it leaves said focal plane in order to reducedissipation of the electrostatic charges on said layer.

5. A xerographic camera comprising:

(a) an optical system including a shutter,

(b) mounting means to mount a multi-exposure roll of prechargedxerographic plate,

(c) a transparent conductive layer serving to reduc the field forcestending to dissipate electrostatic charges on said plate while thecamera is idle,

(d) take-up means to transport said xerographic plate from said mountingmeans behind said layer and through the focal plane of said opticalsystem,

(e) means to urge said layer into pressure contact against the portionof the sensitive surface of said plate lying between said mounting meansand said take-up means,

(f) means actuated by said take-up means to release said pressurecontact during plate transport, and (g) means operably connected withsaid shutter to release pressure contact of said layer during operationof said shutter.

6. A xerographic camera apparatus comprising:

(a) a camera housing,

(15) a lens,

(0) feed means for feeding a xerographic web plate having aphotoconductive insulating layer through the focal plane of said lens,

(d) electrostatic charging means within said housing for applying anelectrostatic charge to said layer, and

(e) means to feed a web of thin conductive foil into continuous contactwith said layer as it leaves said focal plane in order to reducedissipation of the electrostatic charges on said layer.

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